1. Field of the Invention
The present invention generally relates to methods for treating electrical steel strips or sheets and, more particularly, to a method for reducing core losses in light-gage grain-oriented silicon steels involving coating such steels with a stress coating to induce tension stresses therein.
2. Description of the Prior Art
In alternating current equipment and machines, such as magnetic core materials in motors, the use of plain carbon steel may cause the electrical losses to be unduly high. The 1 percent to 4 percent silicon in electrical steels, when used as a replacement for the plain carbon steels, serves to materially lessen the electrical losses which would otherwise occur, and silicon-irons are relatively inexpensive alloy steels.
These silicon steels permit the necessary alternations of the magnetic field without undue energy losses because they possess increased electrical resistance, which diminishes that part of the loss due to eddy currents. Also, while the plain carbon steels may gradually become even worse electrically with time during service, it is found that the silicon steels show relatively little of such an aging effect.
Silicon steels produced in sheet and strip form for electrical use and containing up to approximately five percent silicon, this being the upper limit for commercial materials since brittleness or lack of ductility increases as the percentage of silicon increases, are generally referred to as electrical sheets and strip. "Core loss" is the magnetic property commonly used for grading electrical sheets and strip. Core loss may be defined as that amount of electrical energy converted to heat and dissipated uselessly when magnetic structures are magnetized with alternating current. The lower the core loss of the material, the better is its magnetic quality.
The various commercial grades of electrical sheet and strip are generally sold on the basis of a specified maximum core loss at an induction of either 10 or 15 kilogausses (KG). Core loss values are generally expressed as watts per pound (wpp) at 60 Hz or other frequencies and vary for each grade and thickness of steel since the thickness or gage of a sheet affects the magnetic properties of a given grade of steel.
The beneficial effects on core loss provided by stress coatings on grain-oriented silicon steels of conventional thickness (7 mils or thicker) with .mu.10 levels in excess of 1850 at 50 or 60 Hz are well known in the art (.mu.10 indicating magnetic permeability in an applied field of 10 Oersteds). It is generally accepted that a favorable response to tension on the order of 5% to 10% reduction in core loss is reserved only for steels with .mu.10 levels in excess of 1850 and more commonly in excess of 1880. Such coatings have become a commercial reality since the advent of high-permeability steels in the last 15 years because of the significant beneficial core loss reductions experienced by such steels under tension. Furthermore, such beneficial effects from stress coatings are not known for light-gage (less than 7 mils) conventional grain-oriented silicon steels of relatively poorer .mu.10 at test frequencies above 60 Hz and, particularly, at 400 Hz, which is a frequency oftentimes used in the testing and application of such light-gage steels.
It is therefore an object of the invention to apply tension-inducing stress coatings to light-gage grain-oriented silicon steels to reduce the maximum core loss thereof.
It is a further object of the invention to a apply tension inducing stress coatings to grain-oriented silicon steels having low .mu.10 levels to reduce the maximum core loss thereof.
It is a further object of the invention to provide a method for preparing the surfaces of light-gauge grain-oriented silicon steel products to support the stresses of applied tension-inducing stress coatings in order to prevent spalling of the coatings.
Still other objects and advantages will become apparent in light of the description of the invention presented hereinbelow.